1. Technical Field
The present invention relates to a hybrid construction machine which makes combined use of motive power of an engine and electric power of an electrical storage device.
2. Related Art
The background art of the present invention is described below by taking an excavator as an example.
A general excavator includes: a lower travel body of a crawler type, an upper slewing body provided on the lower travel body so as to be slewable about an axis perpendicular to the ground surface; a working attachment attached to the upper slewing body; a hydraulic actuator; a hydraulic pump which discharges hydraulic fluid for driving the hydraulic actuator; and an engine as a drive source. The hydraulic excavator also includes a generator-motor connected to the engine to perform an electric generator action and an electric motor action; and an electrical storage device which is charged by electric power generated by the electric generator action of the generator-motor. The generator-motor is driven by stored electric power, which is power stored in the electrical storage device, to perform the electric motor action, thereby assisting the engine in driving the hydraulic pump.
The output of the generator-motor is controlled to establish the following respective relationships on the relationship between the output of the engine, the required pump output which is an output required of the hydraulic pump, and the output of the generator-motor (the electric motor output or the generator output):
upon charging, the engine output is equal to the generator output plus the required pump output; and
upon assisting, the engine output plus the electric motor output is equal to the required pump output.
In other words, when the required pump output exceeds the engine output, the output of the generator-motor is controlled so as to make up the shortfall in the engine output by the electric motor output; meanwhile, when the required pump output is lower than the engine output, the output of the generator-motor is controlled so as to charge the electrical storage device by the generator output corresponding to the excess of the engine output.
In this hybrid excavator, with increase in the hydraulic load (required pump output), the fuel flow rate (supply volume) is increased to increase the engine output. In this case, the large rate of the increase in the engine output (the amount of change in the output per unit time) makes the fuel flow rate excessively large to cause incomplete combustion, thus deteriorating the combustion in the engine, for instance, involving black smoke. As means for resolving this problem, Japanese Patent Application Publication No. 2009-216058 discloses technology of limiting the rate of increase in the engine output to a specific limit value. Specifically, according to this prior art, when the required pump output exceeds the upper output limit of the engine as set by the limit value, performed is keeping the engine output at the upper output limit of the engine by causing the generator-motor to perform an electric motor action (assist) using electric power discharged from an electrical storage device.
However, the assistance power of the generator-motor depends on the stored electric power of the electrical storage device and the stored electric power varies with the charge rate and the like; this generates a possibility of an assistance shortfall resulting from limitations of the stored electric power, the shortfall preventing the rate of the increase in the engine output from being restricted to the limit value. In short, the prior art technology of limiting the rate of the increase in the engine output may fail to achieve the object of preventing the combustion from deterioration.